1. Field of the Invention
This invention relates to a vehicle in-situ locating apparatus useful for locating a vehicle such as an automobile which runs mainly on roads or a self-propelling vehicle which runs mainly on a predetermined course.
2. Description of the Prior Art
A method of finding the current location of a running vehicle by the vehicle's driver uses a distance sensor and an azimuth sensor to determine the vehicle's location in terms of relative position with respect to the position of departure. The azimuth sensor used generally is a terrestrial magnetism sensor which is advantageous because it is capable of finding the absolute azimuth and it is inexpensive. However, use of any sensor suffers a progressively falling accuracy due to the cumulative error of the sensor. On this account, the development of map matching is under way, by which the position data collected during the run is compared with the map data and the inferred vehicle location is modified thereby to nullify the cumulative error. However, when a vehicle equipped with a location apparatus based on the terrestrial magnetism sensor runs on such road section as a steel bridge, elevated roadway, tunnel or slope, where the terrestrial magnetic azimuth deviates, an erroneous map matching takes place, which deteriorates the accuracy of location. As a method of preventing this, a technique is used of detecting magnetized or terrestrial magnetism variation areas using a digital map, as proposed in Japanese Patent Unexamined Publication (JP-A) No. 62-144016. FIG. 5 shows the system configuration of this method. A right wheel 501 and a left wheel 502 have a right wheel sensor 503 and left wheel sensor 504, respectively, and their outputs are passed to a processor 506, which also receives the output of a terrestrial magnetism sensor 505. The processor performs map matching for the map data read out of a digital map memory 507. Recorded on the digital map are identifiers of terrestrial magnetism variation areas, and if the inferred location calculated by the processor 506 is within such area, the azimuth output provided by the terrestrial magnetism sensor is determined to be unreliable and the heading direction is calculated by using only the wheel sensors that serve as an angle sensor.
Another conventional technique is a method and apparatus for vehicle navigation, as proposed in U.S. Pat. No. 4,796,191 or Japanese Patent Unexamined Publication (JP-A) No. 61-56910. This technique is based on the fact that the region where the vehicle is possibly situated extends on a two-dimensional ma due to the sensor's cumulative error, and it deals with a profile of equal probability on a two-dimensional plane having equal probability of the presence of the vehicle, and, if the equal probability profile crosses the center line of a road on the two-dimensional map, it judges that the vehicle is possibly running on that road. Thereafter, the system further implements the judgement for the continuity of the road and the collation of the running distance vs. heading direction graph with the distance vs. road direction graph and, if a collation with high coincidence results, moves the inferred location to that road. The system uses two kinds of azimuth sensors, and infers the sensor error from the degree of coincidence between the sensors thereby to determine the equal probability profile.
Among the foregoing conventional techniques, the former system necessitates an angular sensor for the relief of the terrestrial magnetism sensor in a terrestrial magnetism variation area imparted by the map data, and it involves a problem of considerable azimuth deviation if the terrestrial magnetism variation area extends longer and the heading of the vehicle varies progressively while the azimuth sensor is replaced with the angular sensor. The latter system is devised to cope with the above situation without the need of memorizing terrestrial magnetism variation areas as ma data. The main cause of growing azimuth error in using a terrestrial magnetism sensor for the azimuth sensor is the distortion of terrestrial magnetic field when the vehicle is running on a steel bridge, elevated roadway or tunnel. This conventional system uses a pair of wheel sensors and a yaw rate sensor in unison with the terrestrial magnetism sensor so as to correct the anticipated value of azimuth sensor error on the basis of the degree of inconsistency among these sensors. Therefore the system needs two kinds of azimuth sensors, and even with these sensors the anticipated error can be inaccurate. Namely, in case of combining a terrestrial magnetism sensor and a pair of wheel sensors, the wheel sensors have a growing error relative to the distance, resulting in a relatively large detection error. Experience has revealed that when a vehicle was running in a road section which was gradually converted to an elevated roadway, the inconsistency of detected azimuth from the result of the terrestrial magnetism sensor was underestimated even in the locality of distorted terrestrial magnetism, and the system failed to find a road in the expected azimuth and located the vehicle in a parking place instead of a road. In the case of combining a terrestrial magnetism sensor and a yaw rate sensor which developed a growing error with time, the degree of inconsistence of the detected azimuth with the terrestrial magnetism sensor was not estimated properly when the vehicle was running slowly due to a traffic jam or the like on an elevated roadway or in a tunnel even though the zero-point adjustment was conducted at stops of the vehicle, and the system underestimated the error of terrestrial magnetism sensor during a run in a distorted terrestrial magnetism area, failing in the map matching and then resulting in a degraded locating accuracy. Accordingly, problems to be solved include the need of another sensor besides the terrestrial magnetism sensor for constituting a complete azimuth sensor, and, even in this case, the failure of map matching when the vehicle is running on an elevated roadway or in a tunnel where the distortion of terrestrial magnetism is most pronounced, which results in a degraded locating accuracy.